40Ar/39Ar thermochronology of the Pan-African Damara Orogen, Namibia, with implications for tectonothermal and geodynamic evolution

Abstract

Forty-three 40Ar/39Ar step-heating experiments on white mica, biotite, hornblende and whole rocks from the Damara Orogen, Namibia revise and refine the regional perspective on cooling, exhumation, and tectonic reactivation across the orogen. These data also document post-orogenic motion on major shear zones and the thermal effects of late-syn to post-tectonic granite intrusions. These data show regional cooling through temperatures of ∼300–350°C from 545 to 520Ma in the Central Kaoko Zone, 530–510Ma in the Eastern Kaoko Zone, and 495–480Ma in the Southern Zone of the Damara Belt. Discordant 40Ar/39Ar age spectra relate to (1) partial resetting of K-Ar isotopic system in micas by late-stage activity in the major Kaoko Belt shear zones such as the Purros and Three Palms Mylonite Zones, giving apparent ages of 467±6, 481±3 and ∼492Ma, and (2) thermal effects from granitic plutions particularly in the Ugab Domain of the Southern Kaoko Belt (500–475Ma) and in the granite-dominated Central Zone of the Damara Belt (490–460Ma). Phyllites of the lower grade flank regions of the orogen record apparent 40Ar/39Ar mica crystallization ages of ∼517Ma (Eastern Kaoko Zone), ∼526Ma (Northern Zone, Damara Belt), and ∼568–553Ma (Southern Foreland Zone, Damara Belt). The Southern Foreland samples, however, are maximum ages, due to variable influence of detrital mica. Rates of cooling vary between zones within the Kaoko Belt. In the Central Kaoko Belt hornblende, biotite and white mica apparent 40Ar/39Ar ages are similar, suggestive of rapid cooling through the 300–550°C temperature range, whereas in the Orogen Core or Western Kaoko Belt hornblende and biotite apparent ages are older than the white mica ages, because the white micas record the reactivation of the main structures. These data support models of diachronous deformation and metamorphism of the component parts of the orogen, and suggest that Pan-African orogenesis in the Damara is broadly bracketed between 580 and 500Ma with final movements through 480Ma, and spans through the terminal amalgamation of Gondwana in the Kuunga Orogeny through the initiation of subduction under the Gondwana continental margin.